1. Field of the Invention
The present invention relates to a method for fabricating a thin film transistor having a channel region formed of a polysilicon film.
2. Description of the Related Art
A thin film transistor (TFT) including a MOSFET (metal oxide semiconductor field effect transistor) having a channel region formed of a polysilicon film has been widely used as a load element in the memory cell for a complete CMOS (complementary metal oxide semiconductor) type static random access memory (SRAM) or as a switching element in a pixel and a circuit element for the circuit connected thereto for a "driver-monolithic" type liquid crystal display (LCD) apparatus. Since such a TFT is used for loading a memory cell or driving a LCD, characteristics of rapid response speed and a minimized leakage current are required. The leakage current is thought to originate from field emission of carriers through trap states in the grain boundaries of drain junction regions in the TFT. Therefore, in order to provide the TFT with these characteristics, the polysilicon film used for the channel region of the TFT needs to have excellent crystallinity with a reduced number of defects such as localized trap states.
A method for fabricating a TFT having a polysilicon film with high crystallinity has been proposed, for example, in T. Aoyama, et al., "Extended Abstracts of the 22nd (1990 International) Conference on Solid State Devices and Materials", pp. 389-392 (1990).
This method will be described with reference to FIGS. 17 to 19. Referring to FIG. 17, an amorphous silicon film 2 with a thickness of 80 nm is formed on an insulating substrate 1 by low pressure chemical vapor deposition (LPCVD) at 500.degree. C. Then, as shown in FIG. 18, the amorphous silicon film 2 is thermally treated (polycrystallized) in a nitrogen (N.sub.2) atmosphere at 600.degree. C. for 20 hours in order to form a polysilicon film 3. The polysilicon film 3 is patterned to form an active area. Thereafter, as shown in FIG. 19, a gate insulating film 4 made of silicon oxide is formed to cover the polysilicon film 3. Then, a gate electrode 5 made of polysilicon is formed on a portion of the gate insulating film 4. Phosphorus (P) ions are implanted in portions of the polysilicon film 3 using the gate electrode 5 as a mask, thereby to form an n.sup.+ -type source and drain regions 3a and 3b. Finally, another heat treatment in a nitrogen atmosphere is conducted at 1000.degree. C. for one hour so as to activate the implanted phosphorus impurities and to simultaneously repair defects in the polycrystals of the polysilicon film 3.
Another method for fabricating a TFT having a polysilicon film with high crystallinity is described in S. Ikeda, et al., "IEDM 90", p. 469.
This method will be described with reference to FIGS. 20 to 23. Referring to FIG. 20, a gate electrode 5 is formed on a portion of an insulating substrate 1, and the gate electrode 5 and the insulating substrate 1 are covered with a gate insulating film 4 made of silicon oxide deposited by LPCVD. Thereafter, as shown in FIG. 21, an amorphous silicon film 2 with a thickness of 40 nm is formed on the gate insulating film 4 by LPCVD at 520.degree. C. using monosilane (SiH.sub.4) as a material gas. As shown in FIG. 22, the amorphous silicon film 2 is polycrystallized by heat treatment in an oxygen (O.sub.2) atmosphere at 800.degree. C. for 10 minutes so as to form a polysilicon film 3. At this time, the exposed surface portion of the polysilicon film 3 is oxidized to form a silicon oxide film 3c. Then, as shown in FIG. 23, boron (B) ions are implanted in portions of the polysilicon film 3 using an appropriate mask, thereby to form p+-type source and drain regions 3a and 3b. Finally, another heat treatment in a nitrogen atmosphere is conducted at 850.degree. C. for 20 minutes so as to activate the implanted boron impurities and simultaneously to repair defects in the polycrystals of the polysilicon film 3.
However, according to the above-described conventional methods, the polysilicon film 3 is simply thermally treated in the nitrogen or oxygen atmosphere, and the repair of defects in the polycrystals is conducted by use of an annealing effect produced by the thermal treatment. With such a process by annealing, only a limited extent of repair of defects is possible. As a result, a polysilicon film with sufficiently high crystallinity can not be obtained by these conventional methods.